1. Field Of the Invention
This invention relates to a system for drilling a borehole utilizing a downhole drilling motor, and more particularly, but not by way of limitation, to an apparatus for controlling the flow of fluid through a downhole drilling motor specifically to maintain adequate fluid flow through an unsealed bearing section of the downhole drilling motor.
2. Setting of the Invention
To drill an oil or gas well, for example, a drill bit is rotated against the earth at a preselected location to form a borehole intersecting one or more formations which hopefully contain oil or gas. One way to rotate the drill bit is to apply a rotary force at the surface and transmit it through the entire drill string extending from the surface to the bottom of the borehole where the drill bit is located. Another technique is to locate a motor, referred to as a downhole drilling motor, near the lower end of the drill string and to create there a more localized rotary force by pumping drilling fluid through the downhole drilling motor. This precludes having to turn the entire drill string as would be done in the first-mentioned technique.
Downhole drilling motors are well known in the art (see, for example, "Background of the Invention" section in U.S. Pat. No. 4,114,703 to Matson, Jr., et al.). For present purposes, it is sufficient to note that a downhole drilling motor typically has a rotatable drive shaft journaled in bearings. One problem which has resulted from this construction is how to maintain adequate lubrication and cooling of the bearings to prevent premature failure.
One type of downhole drilling motor which tries to maintain adequate lubrication and cooling of its bearings utilizes a sealed bearing chamber filled with lubricant. See U.S. Pat. No. 4,114,703 to Matson, Jr., et al.; U.S. Pat. No. 4,114,704 to Maurer et al.; U.S. Pat. No. 4,225,000 to Maurer; U.S. Pat. No. 4,246,976 to McDonald, Jr.; U.S. Pat. No. 4,324,299 to Nagel; U.S. Pat. No. 4,329,127 to Tschirky et al.; U.S. Pat. No. 4,361,194 to Chow et al.; U.S. Pat. No. 4,372,400 to Beimgraben; and U.S. Pat. No. 4,577,704 to Aumann.
Another type of downhole drilling motor includes an unsealed bearing section through which drilling fluid is to be flowed to try to maintain adequate bearing lubrication and cooling. It is this type of motor with which the present invention is concerned.
The bearings in this "unsealed" type of downhole drilling motor are cooled and lubricated by porting a small fraction (for example, 5-10%) of the drilling fluid through the bearing section of the motor (one flow restrictor construction is shown in U.S. Pat. No. 4,220,380 to Crase et al.). The flow through the unsealed bearing section constitutes a parallel branch of a flow circuit which also includes a main branch typically defined through a drive shaft of the motor and a drill bit connected to the drive shaft. Because the branches are in fluid parallel, the pressure drop across each is the same. Thus, for a given pressure drop, the volume of fluid ported through the bearings is a function of the clearance (that is, effective minimum cross-sectional flow area) in the bearings branch and the clearance in the bit branch.
There is a minimum flow rate that is required to adequately lubricate and cool the bearings in a given downhole drilling motor having an unsealed bearing section. For example, in one type of motor, this flow rate might be achieved when the bit branch pressure drop is about 200 pounds per square inch. If the motor bearings are operated at less than whatever the minimum flow rate is, such as by having too small of a bit pressure drop, the motor bearings will not be adequately lubricated and cooled and will likely fail prematurely.
When operating a downhole drilling motor and a connected drilling bit with the bit off-bottom, it is common practice such that the overall flow rate of the drilling fluid through the drill string is frequently less than when the bit is on-bottom. Also, the branch pressure drop is normally much lower than the minimum pressure needed to divert a sufficient portion of the lower flow rate to lubricate and cool the motor bearings. It is also common practice to utilize in conjunction with downhole motors drag-type drill bits wherein when the bit is off-bottom, there is less restriction within the bit branch flow passage passing through the drive shaft of the motor and on through the drill bit so that a larger percentage of the flow may go through this passage than would occur with the bit on-bottom. This greater flow through the bit branch reduces the flow through the bearing section branch of the fluid circuit. This can be a typical operating condition during a reaming operation. When this occurs, and assuming the drop in flow through the bearing section is large enough, the motor bearings would be run with inadequate lubrication.
Three alternatives exist in such a situation as is brought about in, for example, reaming: (1) trip the drill string and drill bit out of the borehole and ream without the downhole drilling motor; or (2) stay in the hole with the bit off-bottom, run the motor, and risk damaging the bearings; or (3) do something to try to maintain adequate flow through the bearing section. Two specific proposals for trying to maintain adequate pressure or flow are disclosed in U.S. Pat. No. 4,546,836 to Dennis et al. and U.S. Pat. No. 4,560,014 to Geczy. One way of trying to maintain adequate flow which is known to have been used is to run a fixed orifice pressure restrictor between the bit and motor to assure that the pressure drop across the bearings is always adequate.
Of the aforementioned three options, the first is normally not chosen because of economic constraints. That is, it is typically too time-consuming and thus expensive to trip out of the hole. The third option is preferred; but where it has been attempted by using a fixed orifice, it has been undesirable because a fixed orifice wastes hydraulic energy that may be needed to run the motor. As a result, the second option is typically used, whereby premature failures are more likely to occur.
Therefore, within the overall context of a system for drilling a borehole, but specifically with reference only to such a system incorporating a downhole drilling motor having an unsealed bearing section, there is the particular need for an economic flow control apparatus which is variable in that it will constrict the primary flow through the "bit branch" of a motor and a connected drill bit as needed to divert flow through the "bearing branch" (such as when operating off-bottom during reaming) but will allow normal flow therethrough when needed (such as when operating on-bottom). Such an apparatus should be adaptable for connection in-line with existing motors and bits so that redesign or remanufacture of motors and bits is not needed. Where redesign or remanufacture is considered, however, it would be desirable for such a flow control apparatus to be adaptable for incorporation within another component, such as the drill bit. Such a flow control apparatus could be operable one time downhole, but resettable at the surface; or it could be automatically operable and resettable downhole in response to fluid pressure or fluid flow. Although such broad concepts as in-line connection or one-time operability or repetitive resettability individually are not novel, they would be desirable features of a novel and improved flow control apparatus (see, in-line connection within the context of a sealed bearing section downhole drilling motor suggested in U.S. Pat. No. 4,225,000 to Maurer at column 11, line 43 to column 12, line 14; and a resettable dump valve responsive to fluid pressure disclosed in U.S. Pat. No. 4,372,400 to Beimgraben).